2013ApJ...773..127P

We present the identification of the companion star to the intermediate-mass binary pulsar (PSR) J1439-5501 obtained by means of ground-based deep images in the B, V, and I bands, acquired with FORS2 mounted at the European Southern Observatory (ESO)-Very Large Telescope (VLT). The companion is a massive white dwarf (WD) with B = 23.57±0.02, V = 23.21±0.01, and I = 22.96 ± 0.01, located at only ∼0.''05 from the pulsar radio position. Comparing the WD location in the (B, B-V) and (V, V-I) color-magnitude diagrams with theoretical cooling sequences, we derived a range of plausible combinations of companion masses (1 M_☉ ≲ M_COM ≲ 1.3 M_☉), distances (d ≲ 1200 pc), radii ( ≲ 7.810^–3 R_☉), and temperatures (T = 31350_–7400²¹⁵⁰⁰). From the PSR mass function and the estimated mass range we also constrained the inclination angle i ≳ 55° and the pulsar mass (M_PSR ≲ 2.2 M _☉). The comparison between the WD cooling age and the spin-down age suggests that the latter is overestimated by a factor of about 10.